This invention relates to an improvement in a method for control of the feeding of a liquid. In a preferred embodiment it relates to an improvement in a shot size control method in a liquid reaction molding (LRM) system where at least two liquid reagents are mixed in substantially a predetermined ratio and injected into one of a plurality of molds, selected by a programmable controller on a first-come-first-served basis, in connection with which the invention will be described.
LRM systems typically bring together measured amounts of at least two liquid reagents that chemically react to form a solid. A liquid polyol (e.g. a polyester) and polyisocyanate are thus reacted to form a polyurethane. In such a system, the liquids are measured or metered, vigorously mixed en route to the mold, and injected into the mold where they are solidified by curing if a thermoset plastic is being made.
As explained in further detail below, shot size control for liquid reaction molding (synonymous for present purposes with reaction injection molding and liquid injection molding) has been achieved by the use of a positive displacement injection means, in some devices a plunger type pump where the volume swept through by the piston determines the amount of liquid moved. However, the injection termination position (a sort of top dead center) has commonly been fixed at a location having a sufficient clearance volume that the plunger would not physically strike the end of the cylinder in which it reciprocates.. This practice has lead to establishing the starting position of the stroke necessary by first establishing the termination position and then measuring back by the amount required for injection and other events of the cycle.
The prior art practice presents no particular complications as long as all of the shots are the same size, and so long as the reagent conduits have substantially the same elasticity, flow characteristics, pressure drop, etc. However, complicated problems arise if successive shots require substantially different swept volumes for the plunger. Such is the case if different size moldings are made on successive shots and/or if the respective reagent conduits have different hydraulic characteristics such as those mentioned above. For one thing, such a situation (different swept volume requirements) inherently needs different injection stroke each time and therefore requires some sort of program to reset the plunger to a different starting place for each successive stroke.
The present invention is directed to an improved method which facilitates having different plunger strokes on successive strokes. Simply stated the present invention requires resetting the plunger to the same starting position each time. The starting position is so located that there is more than enough volume for its stroke plus the other requirements of the cycle including having a clearance volume sufficient to avoid bottoming out. The termination position is different if the stroke is different. This differs from prior art practice where resetting would be done to a different position each time but would terminate at the same position upon completing the injection stroke.
The present invention is applicable to a wide range of liquid reaction molding systems, and has its preferred application to those systems where one metering system services a plurality of molds each of which may require a different volume of material to fill its mold cavity.